1. Field of the Invention
The present invention relates to a negative electrode active material, a nonaqueous electrolyte battery using the negative electrode active material, a battery pack using the nonaqueous electrolyte battery, and a vehicle having the battery pack mounted thereto.
2. Description of the Related Art
Vigorous research is being conducted on a nonaqueous electrolyte battery in which the battery is charged and discharged by the migration of lithium ions between the negative electrode and the positive electrode in an attempt to develop a high energy density battery.
The nonaqueous electrolyte battery is required to satisfy various characteristics depending on the use of the battery. For example, it is desirable for the nonaqueous electrolyte battery used as a power source of a digital camera to achieve the discharge not lower than about 3 C, and for the nonaqueous electrolyte battery mounted to a vehicle such as a hybrid automobile to achieve the discharge not lower than about 10 C. Such being the situation, the nonaqueous electrolyte battery used in the fields exemplified above is required to exhibit an excellent charge-discharge cycle life when the charge-discharge is repeated under a large current.
The nonaqueous electrolyte battery available on the market nowadays comprises a positive electrode in which a lithium-transition metal composite oxide is used as the positive electrode active material and a negative electrode in which a carbonaceous material is used as the negative electrode active material. In general, Co, Mn, Ni, etc. are used as the transition metals contained in the lithium-transition metal composite oxide used as the positive electrode active material.
In recent years, a nonaqueous electrolyte battery in which lithium-titanium oxide having a high Li ion insertion potential, compared with the carbonaceous material, is used as a negative electrode active material has been put to the practical use. The lithium-titanium oxide is small in change of volume accompanying the charge-discharge operation of the secondary battery, and, thus, permits the nonaqueous electrolyte battery using the lithium-titanium oxide as the negative electrode active material to be excellent in the charge-discharge cycle characteristics, compared with the nonaqueous electrolyte battery using the carbonaceous material as the negative electrode active material. Particularly, it is desirable to use lithium titanate having a spinel structure as the negative electrode active material.
For example, Japanese Patent Disclosure (Kokai) No. 09-199179 discloses a nonaqueous electrolyte battery comprising lithium titanate, which is small in change of volume during the charge-discharge operation of the secondary battery, as the negative electrode active material. It is taught that the nonaqueous electrolyte battery is small in change of volume, and that the short circuiting and the decrease of the battery capacity accompanying the swelling of the electrode are unlikely to take place.
On the other hand, Japanese Patent Disclosure No. 09-309727 refers to secondary particles of lithium titanate having a laminate structure constructed such that a plurality of plate-like or flake-like lithium titanate primary particles are superposed one upon the other. It is taught that pores, each sized about 4 nm (40 Å), are formed among the primary particles to increase the specific surface area of the secondary particles of lithium titanate.